The superenhancer binding factor Med1 is broadly required for optimally expressing and hypermutating IgH genes as well as for terminating their expression through locus suicide recombination

Assembly, expression and maturation of the Immunoglobulin heavy (IgH) chain repertoire necessitates long-range 3D-interactions under regulation by the IgH locus Eµ and 3’ regulatory region (3’RR) super-enhancers. In progenitors, Eµ instrumentally supports VDJ recombination notably towards distal VH genes. In mature cells, the 3’RR first boosts the production of membrane-type IgH transcripts. It later attracts Activation-induced cytidine deaminase at three specific locations: VDJ genes undergoing somatic hypermutation (SHM), switch regions undergoing class switch recombination (CSR), and the 3’RR itself during locus suicide recombination (LSR). In plasma cells devoted to abundant Ig secretion, a late long-range effect is to promote high IgH transcription. The Mediator subunit Med1, which binds super-enhancers and has increased activity after phosphorylation by ERK, was previously attributed a role restricted to class-switched cells. We now show its broader impact on IgH expression, both by promoting recombination of distal VH genes in progenitors and by later supporting SHM. Med1-dependence also marks all long-range functions of the 3’RR, including not only intra- but also trans-chromosomal CSR, LSR, optimal BCR expression and the transcriptional boost featured by plasma cells. Altogether, Med1 appears as a major and specific actor of all the long-range tasks effected by the IgH locus super-enhancers beyond transcription. 3 or 4 RNA-seq point was realized per genotypes (3 for wt and 4 for mutant) from in vitro activated (LPS + IL4) splenic B cells